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| Home > Publications database > Laser processing for the integrated series connection of thin-film silicon solar cells |
| Dissertation / PhD Thesis/Book | FZJ-2016-01728 |
2016
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-119-4
Please use a persistent id in citations: http://hdl.handle.net/2128/9881
Abstract: The integrated series connection of solar cells is an essential aspect for thin-film photovoltaic technology. With a series connection a high output voltage of the module is achieved while the output current is kept low. Thus, Ohmic losses in the contact materials are kept low as well. In thin-film silicon solar technology the steps to create the interconnection are commonly done by laser ablation integrated in-between the depositions of the solar cell layer materials. In three steps laser scribing is used to selectively remove layers locally in the form of lines across the module substrate. In a first step the front-contact is removed for electrical insulation and cell stripe definition. Afterwards, the absorber is removed locally exposing the front-contact beneath. Finally, the interconnection is formed when the back-contact is removed locally as well. The area that is needed for the interconnection of two neighboring cells is no longer active for current generation. Depending on the technology 5-10% of active area is lost. The reduction of this area holds an attractive potential for an increase of the module efficiency. The topic of this thesis is the investigation of the lower geometrical limits for the dead area reduction for substrate side laser processing of thin-film silicon solar cells. It is well-known that the interconnection and the laser processes can have an impact on the performance of the solar module. Therefore, the characterization of the impact on the performance is of special importance when laser processes are used that are capable of generating a reduced interconnection width. P1: for the front-contact insulation process it was found out that the scribe quality strongly depends on the used laser wavelength. Ablation mechanisms that are driven by material phase changes (scribing with 532nm or 1064nm) can lead to smoother scribe edges compared to mechanisms dominated by stress-induced removal (355nm) where non-uniform rip-off at the edges occurs. However, in certain processing regimes, strong ablation debris redeposition in direct vicinity of the P1 scribe is observed when small beam spot radii (<10µm) are used. Such redeposition has a severe impact on the solar cell performance in this region. With proper wet-chemical cleaning the amount of redeposited debris on the front-contact and the negative impact on the solar module can be minimized. Parasitic shunting of two neighboring cell stripes by deposition of absorber material into the P1 scribe increases when the scribe width is reduced. Measurements show that the overall magnitude of the shunt is in a value range that impact on the solar module is negligible for commonly used cell topologies. P2: the width reduction approach was extended for the absorber removal process (P2). To ensure the selectivity of silicon removal without damaging of the front-contact beneath, only 532nm was used for scribing. For this wavelength ablation is strongly assisted by mechanical stresses generated by hydrogen diffusion from the absorber layer and/or thermal expansion of the absorber layer. Mechanical constraints limiting the lower scribe width are found that depend on the absorber thickness and the laser beam spot size. Such behavior can be explained directly from linear elastic fracture mechanics where removal of the layer is determined by the relation between delamination at the interface and fracture of the absorber along the circumference of the spot. It can be concluded that for substrate side laser-induced ablation thin scribe lines are only possible for thin layers. The parasitic series resistance formed by P2 also increases as the scribe width is decreased. However, for processing of amorphous silicon absorbers, with a beam radius 10µm, the minimal achievable resistance value is strongly increased. In fact, much more than what would be expected just by the geometrical contact area reduction. This is most likely owed to changes of the specific contact resistance due to increased debris redeposition within the P2 scribe prior to back-contact deposition. In contrast, such effects are not observed for processing of tandem absorber where debris redeposition is less pronounced. Here, low series resistances, with only minor impact on the module performance, are achieved for all investigated beam spot sizes. P3: the back-contact insulation process (P3) is similar to P2 since the back-contact is removed indirectly by removal of the absorber beneath. Shunting between front- and back-contact can occur at the direct P3 scribe edges. These shunts are possibly formed due to heat generated by sub-threshold energy intake of excess energy from the shoulders of Gaussian distribution of the laser. The mechanical constraints on the minimal achievable scribe widths are even stronger than what was observed for the optimization of the P2 process. This is owed to the additional overall thickness of the layer-stack due to the back-contact. Furthermore, for tandem solar cell processing the scribe edges are strongly distorted by delaminated material while clean edges are obtained for a-Si:H solar cells. The parasitic shunting by P3 scribing increases by many orders of magnitude when a processing beam radius of 10µm is used. However, just like it was observed from P2, an overall weaker deterioration is obtained for scribing of tandem solar cells than for amorphous silicon cells. It is possible that material modifications are more localized in the a-Si:H top-cell. Together with the higher thickness of the tandem cells (300nm vs. 1.4µm) the impact on the whole device is not as pronounced.
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